A brown bear with two cubs looks out of its den in the woods under a large rock in winter.
Enlarge / A brown bear with two cubs looks out of its den in the woods under a large rock in winter.

As days in the north get longer and the snow starts to melt, animals start to awaken from their winter slumber. It is tempting to say that they arewaking up, but it is more complicated and mysterious than that.

Bears are the biggest hibernators of all the animals that are hibernated. The energy and curiosity of preschoolers makes adult bears as big as American football players, but they have no trouble hunkering down for months at a time. Elena Gracheva, a neurophysiologist at Yale University in New Haven, Connecticut, says that shutting down a big creature defies easy explanation.

In the 2020 Annual Review of Cell and Developmental Biology, Gracheva and her co-authors explored the workings of bear hibernation, which offers important insights into the workings of large mammals. A better understanding of the process could change our approach to a wide range of human conditions, including stroke, osteoporosis, Parkinson's disease and Alzheimer's disease.

As the climate warms, bears will have to rethink their concept of being in the wilderness. How they respond will tell a lot about their commitment to winter naps and their relationship with the climate.


Not cool

Bears take a different approach to sleeping than other slumberers. The ground squirrel can temporarily drop their body temperature to -2 degrees C. Bears don't lose any heat in their winter dens, but they still qualify as hibernators because their metabolism slows to a crawl. It is a process that Brian Barnes, a zoologist at the University of Alaska Fairbanks, and his colleagues carefully tracked for more than a decade.

Oxygen intake and carbon dioxide production were important measures of metabolism and the winter quarters had chambers that could measure them. It was the first study to show that animals can sleep.

Humming bears aren't just resting away the winter. He says that they only get up to switch sides for six months when they shut down.

The level of lethargy would cost humans. Our bones would thin after months of being inactive, even if we were recovering in a hospital bed or riding a rocket to Mars. Bears don't have a problem. The secret to their strong bones is coming to light. In 2021, Barnes and colleagues published a study showing that bears are able to shut down genes involved with the breakdown of bone.

It might be possible to manipulate the same process in people to prevent osteoporosis, according to the researchers. Barnes says that such an approach could be helpful for people who are confined to extended bed rest.


The sluggish metabolism of bears is an amazing feat. A bear can slow its breathing and heart rate by up to 75 percent for months at a time. Gracheva says the strategy makes good sense, even though nobody knows how they put on the brakes. She thinks bears don't chill out like ground squirrels because it would take too much energy to warm up their large bodies in the spring. Instead, they curl up, letting their fat and fur keep them warm with just a few occasional shivers to help keep blood moving.

The mastery of near suspended animation by human-sized (or bigger) animals has caught the attention of science fiction writers and others who dream of someday being able to send astronauts around the solar system.

Common or Hazel Dormouse, <em>Muscardinus avellanarius<em>, hibernating in nest, autumn, Norfolk UK.
Enlarge / Common or Hazel Dormouse, Muscardinus avellanarius, hibernating in nest, autumn, Norfolk UK.

Barnes explains that heart attacks and strokes reduce the supply of oxygen and vitamins to the brain. If doctors could quickly reduce the demand by putting a patient in a state of sleep, it would be less damaging. The first hour after a stroke is when stroke victims are most likely to benefit. Doctors call their window of opportunity the golden hour, if they could replicate hibernation to a point where the brain doesn't need as much blood.

In March 2021, researchers in the United States and China proposed a study that would use a combination of the sedative drug promethazine and the antipsychotic drug chlorpromazine.